I feel like I must be running a whole different beta compared to some of you. I am on a 2013 MBA 11". I was running the latest 10.9.3 developer release. I just downloaded and upgraded to Yosemite the moment it became available. I've had absolutely no problems. I think the icons look good (personal opinion, I know) and the new font looks great. I really have no big problem seeing anything on my screen--maybe it's the retina screens that are hard to see?
My apps all work so far including: Scrivener, MS Office 2011 (including Outlook), iStat Menus, DropBox, iWork, iTunes, Chrome & Firefox. Both my printers (a networked Brother laser and a networked HP AIO) work fine. I am not sure why some of you are having issues. I've not noticed any lag other than it seems the notification center can stutter a tiny bit the first time I slide it open after it sleeps or I restart. Also, I can move my icons around in the dock (somebody earlier said they couldn't.)
Maybe a fresh install would help for those of you having issues? Though, again, I did not do a clean install; I just upgraded what I was on.
My only complaints so far are, the tiny dots that show which apps are open and the SpotLight window appearing in the center of my screen--I liked it better in the corner.